This invention relates to lenses for implantation in aphakic eyes and, more particularly, to a novel and highly-effective intraocular lens that makes it possible to correct for errors in centration and tilt that have now been found to be statistically different in the case of conventional implantation in right and left eyes, respectively. The invention relates also to a method by which normal vision can be more nearly restored to aphakic eyes.
Surgical removal of the crystalline lens of the eye because of various medical problems including notably the development of cataracts has become commonplace. The crystalline lens is replaced with an intraocular lens (IOL) equipped with haptics for maintaining the IOL in a predetermined position in the eye. Various mounting locations for the IOL have been proposed, including the anterior and posterior chambers, and within each chamber various placements of the haptics have been proposed. For example in the case of an IOL implanted in the posterior chamber, both haptics can engage the ciliary sulcus or the capsular bag (symmetric fixation) or one haptic can engage the ciliary sulcus and the other the capsular bag (asymmetric fixation).
A U.S. Pat. No. 4,235,200 to Kelman discloses various examples of IOLs designed to compensate for the tilting of the lens that otherwise occurs when one haptic, for example the superior haptic, engages the ciliary sulcus and the other (inferior) haptic engages the capsular bag. In accordance with the teaching of the patent, one of the haptics is provided with a portion that is either stepped or angled in order to eliminate the tilt that would otherwise result from the asymmetric fixation of the haptics. Also, the haptic engaging the ciliary suclus is either longer or less flexible than the haptic engaging the capsular bag in order to compensate for the decentration that would otherwise result from the asymmetric fixation.
In accordance with this and other teachings of the prior art, however, as exemplified by U.S. Pat. Nos. 2,834,023, 4,087,866, 4,316,292, 4,327,450, 4,328,595, 4,441,217, 4,535,488, 4,601,721, 4,657,546, 4,657,547, 4,704,123, 4,718,904, and 4,734,095 and Phillips, P. et al., Measurement of intraocular lens decentration and tilt in vivo, J. Cataract Refract. Surg. 14:129-135, 1988, decentration and tilt, to the extend that they are recognized as problems at all, are considered to be independent of the "handedness" of the eye in which the IOL is implanted, and an IOL intended for implantation in the OD (oculus dexter or right eye) is no different, except of course for the refractive correction, from an IOL intended for implantation in the OS (oculus sinister or left eye).
Is now been found through careful measurements made on scores of patients with implanted IOLs that there is a systematic, statistically significant difference between the errors in centration and tilt that accompany implantations of IOLs in left eyes and those that accompany implantations of IOLs in right eyes.
Specifically, measurement of posterior chamber intraocular lenses, all implanted by the same surgeon, in a series of 103 eyes has shown that lenses tend to decenter superotemporally and to tilt with the superonasal edges tipped forward. Decentration induces prism and other optical aberrations and can result in exposure of edges and positioning holes within the pupil. Average decentration in the 103 eyes was 0.68 millimeters. Tilt produces astigmatism and coma that increases with the square of the tilt angle. Average tilt was 6.6.degree., an amount that produces less than 0.25 diopters of astigmatism. By comparison, 15.degree. of tilt produces one diopter of astigmatism.
It is generally considered advisable to center the intraocular lens on the pupil. The intraocular lens is conventionally radially symmetrical so that if the lens is centered on the pupil the optical center of the lens is on the line of sight of the aphakic eye. Such placement of an intraocular lens does not optimally restore the optical alignment of an aphakic eye if, as is often the case, the visual axis of the aphakic eye is not coincident with the line of sight. Moreover, the surgeon cannot accurately judge where the center of the pupil (or line of sight) is, especially since the center of the pupil often migrates slightly when the pupil is pharmacologically dilated in preparation for surgery.